Well I'm replacing a shed in my backyard. It has a cement foundation already that is roughly 12'x20'. I don't want anything fancy or anything, just a basic wood shed. I also want to put in a pull up garage door (no motor though). It sounds really easy and I have an idea how, I just want some suggestions and maybe a rough estimate of how much wood i will use on the frame

If you go to Lowes or Home Depot they can design one right on the computer and come up with a material list.
Make sure to use Adavntec for the sub flooring. Far less likly to rot out.
If you go with wooden siding like T-111 use solid stain not paint so it does not start peeling off after the first year.
DO not have the siding within 6" of the slab or ground or it will rot out.
Install a ridge vent to reduce heat inside and help the shingles last longer.
Have at least a 6" over hang on the roof on all sides and the siding will last longer.